Many electronic systems comprise a plurality of functional units interconnected by a common communication medium. An example of such systems is a typical computer which generally comprises a plurality of peripheral units interconnected with a processing unit by a communication bus that is common to all units. In many such systems, and particularly in computers, some means must be provided by which a central administrative unit, such as the processing unit, can access another functional unit, such as a peripheral circuit pack (also referred to as a peripheral circuit board), individually, to the exclusion of other units, over the common medium. Clearly, a way to accomplish this is to provide a means for logically distinguishing between the units, such as by configuring each unit to respond to a particular unique I.D. or address spectrum.
However, it is desirable to have systems configurable and reconfigurable by either system users or automatically by the system itself. In such a case, the problem arises of how to individually access the units in order to configure them, that is, in order to assign to them the logically-distinguishing characteristic, such as a unique I.D. or address spectrum, in the first place.
Many techniques for doing this are known. For example, each unit may be equipped with I.D. or addressspectrum-defining switches or straps that are manually settable by the user. However, this technique has reliability problems because it is dependent on user dexterity, accuracy, understanding of system configuration, and ability to follow instructions. Also, it is problematical in that it does not allow for automatic system configuration or reconfiguration. Another technique involves the use of backplanes, signalpin staging fields, cables, or other means by which leads, contacts, or other electrical and/or mechanical connections that are dedicated to a particular unit may be connected only to that unit. However, by definition, this technique undesirably requires that different connections be made to different units. Thus, a problem of this technique is that it is not suitable for use with buses such as stack buses wherein all bus connections are made in common to all units.